1. Field of the Invention
The present invention relates to liquid crystal displays, and more particularly to methods for forming a stable alignment layer created by ion beam irradiation of a carbon film.
2. Description of the Related Art
Liquid crystal (LC) material employed in liquid crystal displays typically rely on alignment layers to establish a stable pretilt angle and other parameters such as anchoring energy for the liquid crystal material. Typically, the alignment of the liquid crystals for flat panel liquid crystal displays (LCD) is accomplished by placing a thin film of LC material on a mechanically rubbed polyimide film coated on a suitable substrate. Limitations imposed by the mechanical rubbing method (e.g., creating multiple domains for improving the viewing angle) in conjunction with the difficulty of optimizing polymer materials (e.g., polymers that avoid image sticking) make it highly desirable to use alternative materials and a non-contact LC alignment method.
There are a number of different methods/materials which have been shown to create LC alignment besides rubbing, for example, a stretched polymer, a Langmuir Blodgett film, a grating structure produced by microlithography, oblique angle deposition of silicon oxide, and polarized ultraviolet (UV) irradiation of a polymer film.
Non-contact methods to replace rubbing are described in commonly assigned U.S. Pat. No. 5,770,826, which describes a particularly attractive and versatile LC alignment process based on ion beam irradiation of a polyimide surface. The method places the LCs on a polyimide surface which has been bombarded with low energy (about 100 eV) Ar+ ions. This process has many characteristics which make it suitable for the manufacture of LC displays. This method has been extended to include diamond-like carbon (DLC), amorphous hydrogenated silicon, SiC, SiO2, glass, Si3N4, Al203, CeO2, SnO2, and ZnTiO2 films as described in commonly assigned U.S. Pat. No. 6,020,946. Another method for creating an LC alignment layer in a single deposition process has been described in commonly assigned U.S. Pat. No. 6,061,114.
Ion-beam treatment on DLC films (IB/DLC) for the alignment of liquid crystals has many advantages over conventional rubbed polyimide alignment, such as, non-contact processing, alignment uniformity, etc. Usually, DLC films of about 50 angstroms thick are deposited by plasma enhanced chemical vapor deposition (PECVD), and followed by Ar ion beam irradiation. It is believed that the Ar ion beam destroys the amorphous-carbon rings which have a large collision cross section to the ion beam. The amorphous-carbon rings which have a small or zero collision cross section to the ion beam are preserved. The average direction of the remaining carbon rings align the liquid crystal and generate a pretilt angle. The pretilt angle of IB/DLC alignment is not stable. The pretilt angle tends to decrease when the IB/DLC substrates are in contact with moisture or other components in air. The pretilt angle decreases as a function of storage time in vacuum-sealed LC cells with IB/DLC alignment. In addition, the pretilt angle is not stable under ultra-violet (UV) or violet irradiation. After ion-beam treatment, the surface of the DLC films are very active due to the ion-beam induced free radicals on the DLC surface. These free radicals tend to react with many chemical species in contact with them. This reaction may change the surface chemistry of the DLC film or change the orientation the carbon rings. As a result, the pretilt angle will degrade.
Therefore, a need exists for a non-contact alignment layer with a stable pretilt angle for use with liquid crystal displays.